1. Field of the Invention
The present invention relates to methods and system associated with ripple detection.
2. Background Art
As is well known, a DC motor comprises a series of coils or inductors wound around a core. With two fixed contacts called brushes, one of the inductors is connected to the polarization voltage, depending on which of the inductors is in contact with said brushes. Each inductor makes contact with the brush during a specific rotation angle of the motor, and then the brush polarizes another inductor.
Known ripple counting techniques are based on the experimental observation of the fact that when the motor rotates and the contact or brush crosses from one pole to the other, a peak occurs in the current curve of the motor, which peak is easily observable by means of a current sensor. By measuring the time elapsed between two of these peaks, the motor rotation period and also the position of the movable element actuated by the motor can be calculated, thereby being able to calculate the instantaneous speed of said movable element. This ripple detection technique will thus permit controlling the position of a movable element, such as an automotive vehicle window, actuated by a DC permanent magnet multi-polar electric motor, with no need to have a positioning sensor or speed sensor.
The ripple is a feature of the current intensity passing through a DC permanent magnet motor and is closely related to the motor rotation, each time the brushes switch from one commutator bar (rotating commutator segment) to another. Current oscillation detection will permit knowing the displacement of the movable element actuated by the electric motor and following up on its position.
Current rippling occurs basically because of the overlapping of two effects. The first one originates in the counter electromotive force induced in the coils or inductors which, assuming that the rotor of the motor were submerged in a uniform magnetic field, makes said counter electromotive force induced in each coil have a rectified sinusoid shape, that is, rippling occurs in the counter electromotive force generated in the entire winding, and this in turn causes rippling in the current intensity which reaches the motor. The second effect is related to the number of coils or inductors contributing at all times to the total counter electromotive force.
In this aspect, two situations can be shown, a first one in which all the coils contribute, and a second one in which there are two short-circuited coils, and therefore, since current does not pass through them, they do not contribute. The first situation occurs when each one of the brushes is in contact with a single commutator bar or rotating commutator segment, which situation is shown in FIG. 1b, then coils d, c, b, a, l, k conduct half of the current, and coils e, f, g, h, i, j conduct the other half, as can be seen in the diagram. However, when each one of the brushes are in contact with two commutator bars, as shown in FIG. 1a, the second situation occurs, the ends of coils j and d are short-circuited, which causes the counter electromotive force generated in coils j and d to not contribute to the total.